1. Field of the Invention
The present invention relates to drilling fluid telemetry systems and, more particularly, to a telemetry system incorporating a rotary valve for modulating the pressure of a drilling fluid circulating in a drill string within a well bore.
2. History of the Prior Art
Drilling fluid telemetry systems, generally referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, salinity, direction and deviation of the well bore and bit conditions. Other parameters include logging data such as resistivity of the various layers, sonic density, porosity, induction, self potential and pressure gradients. This information is critical to efficiency in the drilling operation.
One example of a prior mud pulse system of the aforesaid variety is illustrated in U.S. Pat. No. 3,964,556. The principles set forth therein require that circulation of drilling fluids be ceased in order to operate the system. Other systems have used a controlled restriction placed in the circulating mud stream and are commonly referred to as positive pulse systems. With mud volume sometimes surpassing 600 gpm and pump pressures exceeding 3000 psi, the restriction of this large, high pressure flow requires very powerful downhole apparatus and energy sources. Further, the systems must require the movement of valve parts under extremely high pressure conditions. This condition results in a myriad of problems relating to the durability of the valve parts subjected to the high pressure, abrasive, fluid flow conditions.
Another example of a prior mud pulse system is illustrated in U.S. Pat. No. 4,351,037. This technology includes a downhole valve for venting a portion of the circulating drilling fluids from the interior of the drill string to the annular space between the pipe string and the borehole wall. Drilling fluids are circulated down the inside of the drill string, out through the drill bit and up the annular space to the surface. This circulation pattern develops a pressure differential of about 1000 to 3000 psi across the drill bit. In like manner, a substantial pressure differential exists across the wall of the drill string disposed above the drill bit. By momentarily venting a portion of the fluid flow out a lateral port above the bit in the drill string, a instantaneous pressure drop is produced and is detectable at the surface to provide an indication of the downhole venting. A downhole instrument or detector is arranged to generate a signal or mechanical action upon the occurrence of a downhole detected event to produce the above-described venting. The downhole valve which is disclosed is defined in part by a valve seat having an inlet and outlet, and a valve stem movable to and away from the inlet end of the valve seat and in a linear path with the drill string.
A major problem associated with negative pressure pulse systems is the wear and replacement of valve parts, particularly as the data rate is expanded. It is highly desirable to operate such a system as long as possible since replacement of system components typically requires the time consuming and expensive removal of the valve system from its downhole location and from the drill string at the well head for replacement of the worn parts.
Prior art systems incorporating poppet valves exhibit deletrious wear due to the circuitous flow path of fluid through the valve. The seat of the poppet is worn rapidly by high rates of abrasive fluid flow when the valve is in the open position. Further the design of the poppet is such that a pulse is created only when the valve is open and, therefore fluid flows around and also abrades the valve stem. In addition, it is desirable to have a fast acting opening and closing movement of the valve parts in order to create a sharp pressure pulse for adequate detection at the surface. Rapid closing of the poppet valve generates a high valve head impact force on the valve seat. This force rapidly wears the valve parts, particularly when abrasive particles are present in the fluid flow through the valve. Such particles become impacted in the valve parts and deteriorate the sealing surfaces of the valve. The repeated impact forces may also break portions of the valve parts because erosion resistant materials are generally brittle and not impact resistant.
In view of the disadvantages of poppet valve designs, other valve systems have been improved. Another negative pulse system of prior art design employs a rotary acting valve which utilizes a mass of rotational valve parts. A drive motor and gear system is incorporated to operate the rotational valve head for registration of flow apertures. While effective in reducing abrasive wear the valve actuation through a motor and gear train is relatively slow which reduces pressure pulse definition.
The aforesaid examples illustrate some of the critical considerations that exist in the application of a rapidly acting valve to a high pressure fluid flow for generating a sharp pressure pulse. Other considerations in the use of these systems for borehole operations involve the extreme impact forces and vibrational energies existing in a moving drill string. The result is excessive wear, fatigue, and failure in operating parts of the system. The particular difficulties encountered in a drill string environment, including the requirement for a long-lasting system to prevent premature malfunction and replacement of parts, require a simple and rugged valve system
One advance in mud pulse telemetry systems is shown in co-pending application No. 460,461, filed Jan. 24, 1983 and assigned to the assignee of the present invention. A linear shear valve is disclosed therein which overcomes many of the disadvantages of the prior art and is an excellent overall system for most mud pulse telemetry applications. However, for certain applications requiring higher pulse amplitudes and thus greater valve flow rates the linear acting shear valve exhibits certain limitations. For example, the maximum fluid flow rate and amplitude possible with a linear acting shear valve is limited by the size of the valve orifice which can be opened and closed within given power parameters. The force available for operating the valve gate is limited by the dimensions of the linear solenoid which can be housed within a borehole sub. Because the shear valve actuation is a marked improvement over prior art designs, it would be an advantage to provide the advantages thereof with the capacity of greater valve flow rate and higher pulse amplitude.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a new and improved mud pulse telemetry system utilizing an improved, rotary acting shear valve. The advantages of shear valve actuation are thus provided with a rotary solenoid system controlling a rotary valve gate and seat having a greater cross sectional flow configuration. The rotary solenoid valve also permits a tailoring of the force curve of the solenoid for maximum force over the required distance of movement for actuation of a larger flow valve.